staging: iio: accel: fix error check

commit ef3149eb3ddb7f9125e11c90f8330e371b55cffd upstream.

sca3000_read_ctrl_reg() returns a negative number on failure, check for
this instead of zero.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>